Apparatus and method for generating controlled-linewidth laser-seed-signals for high-powered fiber-laser amplifier systems

ABSTRACT

Apparatus and method for generating controlled-linewidth laser-seed-signals for high-powered fiber-laser amplifier systems. In some embodiments, the natural chirp (frequency change of laser light over a short start-up time) of a DBR laser diode when driven by pulsed current is used to broaden the linewidth of the laser output, while adjusting the peak current and/or the pulse duration to obtain the desired linewidth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention claims benefit of U.S. Provisional Patent Application60/699,894 filed Jul. 15, 2005, which is hereby incorporated byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

A portion of this invention was made with Government support undercontract awarded by the U.S. Government. The Government has certainrights in the invention.

A portion of this invention was made with Government support undercontract #N68936-03-C-0098 awarded by the U.S. Navy. The Government hascertain rights in the invention.

FIELD OF THE INVENTION

The invention relates generally to lasers and nonlinear opticalfrequency conversion and more particularly to methods and apparatusapplicable for generating controlled-linewidth laser-seed-signals forhigh-power fiber-laser amplifier systems.

BACKGROUND OF THE INVENTION

The broad gain bandwidth of conventional fiber-laser systems allows foroperation over a wide range of wavelengths, or even tunable operation.For the simplest fiber laser system with cavity mirrors having broadreflectivity, the output wavelength can be very broad and can vary withpump power, fiber length, and/or other parameters. In some cases, afiber Bragg grating (FBG) can be used as a narrow-band reflector tolimit the linewidth of the fiber laser system, but the range oflinewidths that can be generated is limited, and methods and apparatusto extend the available range are not straightforward. For instance, theminimum bandwidth that can be generated with an FBG is typically on theorder of 6-10 GHz for standard fiber and is even larger forpolarization-maintaining (PM) fiber. An additional complication is thatthe fiber laser system using an FBG typically will operate only near thepeak reflectivity, resulting in a laser linewidth that can besubstantially less than the FBG bandwidth. Alternatively, nonlineareffects in the fiber can broaden the laser linewidth to be substantiallygreater than the FBG bandwidth, particularly for high-peak-powerpulsed-fiber lasers, or even continuous-wave (CW) lasers that can oftenexhibit noisy, unstable output.

The power that can be generated from fiber lasers and fiber-laseramplifiers can often be limited by nonlinear optical effects in the gainand/or delivery fibers used in the system. In particular, StimulatedBrillouin Scattering (SBS) is a well-known phenomenon that can lead topower limitations or even the destruction of a high-power fiber-lasersystem due to sporadic or unstable feedback, self-lasing, pulsecompression and/or signal amplification.

There is a need for laser systems, particularly fiber-laser-amplifiers,where the linewidth of the emission to be generated must be engineeredto lie within a certain range of values. This need can arise forinstance, when a fiber-laser system must produce optical wavelengthsthat only lie within a narrow linewidth, e.g. for coherent detection,coherent phasing of multiple systems, or bandwidth acceptance ofnonlinear optical processes. On the other hand, narrow linewidth canlead to some types of nonlinear optical effects in the gain or deliveryfiber of the system, limiting the peak power that can be generated insuch a system.

The optimum seed source for a fiber amplifier system would be stable,low-noise and produce a given linewidth as required for the particularapplication. If polarized output is required from the system, itspolarization properties must be much better than the requirements forthe output as well. The simplest, most robust, seed sources typicallyused are Fabry-Perot laser diodes or fiber lasers. These are multi-lineand in the case of the laser diodes, the output can extend to several nmor more. FIG. 1 shows an output spectrum for a Fabry-Perot laser diodecentered around 1060 nm. On this scale, the mode structure is apparent,with “gaps” in the spectrum. Multi-longitudinal mode fiber laserstypically have much narrower mode spacing and may operate on a muchsmaller number of modes, but the phenomenon is similar.

FIG. 1 shows an emission spectrum from a Fabry-Perot laser diode showingmulti-longitudinal mode emission over a wavelength range ofapproximately 3 nm (˜900 GHz).

One issue with this type of seed source is that the spectraldistribution of power is not constant, but the power can fluctuatebetween the modes. This is known as “mode partition noise” and thetimescale for redistribution of the spectral power can be on the samescale as the SBS build-up time of a few nanoseconds (e.g., about 5 to 10ns). This can lead to the occurrence of SBS in the fiber amplifiersystem, even though the average linewidth of the seed source may besufficient to avoid the SBS.

Single-frequency laser diodes or fiber lasers avoid the problem of modepartition noise and can be used for a number of applications.Particularly for situations where very narrow linewidth is required,these offer linewidths on the order of a few MHz (diodes) down to tensof kHz (fiber lasers). However, this linewidth is more narrow thanactually needed for some applications and worsens nonlinear fibereffects such as SBS. For instance, the bandwidth acceptance forfrequency doubling can be ten's of GHz so a linewidth narrower than abroad-band free-running fiber laser may be required but a singlefrequency source may be much narrower than actually needed and couldlead to lower power or instabilities due to fiber nonlinearities.

What are needed are improved seed sources that help avoid theabove-described problems and provide other benefits.

SUMMARY OF THE INVENTION

In some embodiments, the invention provides methods of generatingpolarization-maintaining, controlled-linewidth seed signals forhigh-power fiber-amplifier systems.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph 100 of intensity versus wavelength for an exemplaryseed signal.

FIG. 2 is a block diagram of a controlled linewidth seed source 200based on spectral filtering of ASE.

FIG. 3 shows a schematic diagram of a seeded ASE source 300 having anFBG at one end of the ASE fiber source to “seed” a given wavelength.

FIG. 4 shows a schematic diagram of a seeded ASE source 400 havingspectral filtering using a bulk diffraction grating spectral filter anda circulator.

FIG. 5A shows a schematic diagram of a seeded ASE source 500 withdouble-pass amplifier using FBG filter with circulator.

FIG. 5B shows a schematic diagram of an improved seeded ASE source 501with double-pass amplifier using FBG filter with circulator.

FIG. 6 is a graph of the optical output pulse for an exemplary DBR laserdiode such as a Sacher Diode (e.g., one from Sacher Laserteknik,Germany, such as Model DBR-1080-80 or DBR-1060-100) driven with a pulsedcurrent.

FIG. 7 is a graph of the output wavelength of the Sacher Diode operatingin pulsed mode with 200 MA peak drive current.

FIG. 8 is a graph of the output wavelength of the Sacher Diode operatingin pulsed mode with the peak drive current set at 75 MA.

FIG. 9 is a graph of the photodiode signals for the power monitor andthe wavelength monitor after transmission through an etalon. The graphindicates that the laser diode is operating at one wavelength for thefirst ˜5 ns, then jumps briefly to a second wavelength that is nottransmitted, and then to a third wavelength that is transmitted.

FIG. 10 is a graph of photodiode signals for the power monitor and thewavelength monitor after transmission through an etalon.

FIG. 11 is a graph of the scanning Fabry-Perot Signal for the SacherDiode operating in pulsed mode with the peak drive current set at 75 MAand a pulse duration of 35 ns.

FIG. 12 is a schematic block diagram of a narrow linewidth seed sourcewith a tunable linewidth.

FIG. 13 is a graph of diode drive current and relative timing of theAOM.

FIG. 14 is a set of graphs which resulted from scanning Fabry-Perottraces of the seed source linewidth for various AOM delays.

FIG. 15 is a graph of seed source linewidth as a function of the AOMdelay after the laser diode current pulse.

FIG. 16A is a simplified schematic of a pulsed current source thatgenerates a very fast current pulse, where the current magnitude variesas a function of time, increasing the current later in the pulse tocompensate for the reduction in gain over time in a fiber gain medium.

FIG. 16B (which includes 16B1 and 16B2 and 16B3) includes a detailedschematic and a simplified schematic of a pulsed current source thatgenerates a very fast current pulse, where the current magnitude variesas a function of time, increasing the current later in the pulse tocompensate for the reduction in gain over time in a fiber gain medium.

FIG. 16C is a graph of the shape of a shaped laser diode current pulsefor a seed source as a function of time, for five different pulse typesand current magnitudes.

FIG. 17 is a graph of the spectral trace of a seed-source laser drivenby each of the five pulses of FIG. 16C.

FIG. 18 is a graph of gain-switched spike as a function of time.

FIG. 19 is a graph of the shape of a shaped laser pulse for a SOA+DFBseed source as a function of time.

DETAILED DESCRIPTION

Although the following detailed description contains many specifics forthe purpose of illustration, a person of ordinary skill in the art willappreciate that many variations and alterations to the following detailsare within the scope of the invention. Accordingly, the followingpreferred embodiments of the invention is set forth without any loss ofgenerality to, and without imposing limitations upon the claimedinvention.

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

The leading digit(s) of reference numbers appearing in the Figuresgenerally corresponds to the Figure number in which that component isfirst introduced, such that the same reference number is used throughoutto refer to an identical component which appears in multiple Figures.Signals and connections may be referred to by the same reference numberor label, and the actual meaning will be clear from its use in thecontext of the description.

Stimulated Brillouin Scattering (SBS) can lead to power limitations oreven the destruction of a high-power fiber-laser system due to sporadicor unstable feedback, self-lasing, pulse compression and/or signalamplification. SBS can be triggered by a seed source having a wavelengthbandwidth that is sufficiently narrow (e.g., about 50 MHz).

One way to generate output with more controlled attributes is to use amaster-oscillator power-amplifier (MOPA) architecture. The oscillatorcan be optimized to generate a laser seed signal having the appropriatecharacteristics, such as linewidth, and the power amplifier is used toincrease the output power and/or pulse energy to much higher levels.

In some embodiments, the present invention generates a controlledlinewidth that depends on the actual value required. In someembodiments, the present invention provides a seed source based onspectral filtering of an amplified-stimulated emission (ASE) sourceusing fiber Bragg gratings (FBG) that was developed and demonstrated forlinewidths greater than approximately 10 GHz. In some embodiments, thepresent invention provides a spectral filter using a bulk diffractiongrating, allowing for filtering down to a few GHz, using a similartechnique. For linewidths narrower than this, but greater than the fewMHz linewidths of a single longitudinal mode laser diode, someembodiments of the invention provide methods using the wavelength shiftthat occurs during rapid turn-on of a laser diode, or “chirp”. While theASE source can be used for pulsed or cw (continuous wave) applications,in some embodiments, the chirped single frequency diode source islimited to pulsed applications, although, in some embodiments, aplurality of chirped diode sources that have pulsed, chirped outputsthat are staggered over time can be multiplexed together in order toprovide a composite output signal that is cw or nearly cw.

Spectrally-Filtered ASE Source

A number of tests have been performed with different Yb-doped fiber andlengths to investigate their possible use as an ASE source. In someembodiments, fibers (e.g., in some embodiments, based onalumina-silicate glass) that are doped with ytterbium (Yb) to provide ahigh gain at a wavelength of about 1030 nm to 1080 nm. Some embodimentsuse a bulk diffraction grating to generate a 0.1 nm linewidth (about 25GHz). Other embodiments use one or more FBGs (Fiber Bragg Gratings) witha 0.1 nm linewidth in either polarization-maintaining (PM) orHI1060-type fiber in place of the bulk diffraction grating.Optimizations of some embodiments start with a very low power cw signalthat is amplified to about 100 mW. In some embodiments, the signal isthen modulated using an acousto-optic modulator to produce pulses of 100ns to 400 ns duration at a PRF (pulse repetition frequency) of 10 kHz.Finally, the pulsed seed signal is amplified in several more YDFA(Yb-doped fiber amplifier) stages.

FIG. 2 is a block diagram of a controlled-linewidth seed source 200based on spectral filtering of ASE. In some embodiments, source 200includes an ASE source 210, a Yb-doped fiber amplifier (YDFA) 220 havingspectral and polarization filtering, an amplitude modulator 230 (which,in some embodiments, generates flat-topped pulses of about 100 nsduration and at a rate of 10K/second), a YDFA 240 having spectral andpolarization filtering, and a YDFA 250 as a final preamplifier stage. Insome embodiments, the 100 ns pulses from amplitude modulator 230 have bythis stage, been shortened to about 20 to 30 ns by pulse steepening(described below). In some embodiments, the output from stage 250 isamplified by a large-mode-area (LMA) power amplifier as desired. In someembodiments, the LMA power amplifier is a high-gain amplifier that isotherwise prone to SBS.

FIG. 3 shows a schematic diagram of a seeded ASE source 300 having anFBG 340 at one end of the ASE fiber source to “seed” a given wavelength.In some embodiments, source 300 can be used as ASE source 210 of FIG. 4.In some embodiments, source 300 includes a laser diode pump 310 (e.g.,in some embodiments, a 180 mW, 975 nm laser diode) followed by a fiberBragg grating 312 to lock stability, and a fiber coupler 314, and theoutput goes into single mode (SM) pump/signal multiplexer (MUX) 330. Insome embodiments, MUX 330 has as its other left-hand port, aangle-cleaved fiber end 320, which together with MUX 330, helps minimizereflections that could cause lasing in Yb-doped fiber 350 (in someembodiments, a 55-dB reduction is reflection is obtained this way).Fiber Bragg grating (FBG) 340 at the left end of the ASE fiber source(Yb-doped fiber 350) is used to “seed” a given wavelength (e.g., in someembodiments, it is highly reflective (HR) at 1080 nm with a linewidth ofabout 0.16 nm, thus providing enough feedback to fiber 350 (which has again width from about 1030 nm to 1080 nm, depending on fiber length andpump power) to amplify at 1080 nm with a linewidth of about 0.16 nm, butthis and the other parts are adjusted, if needed, to prevent lasing infiber 350. In some embodiments, the output of fiber 350 is passedthrough a non-polarization-maintaining isolator 360 and coupled to thenext stage (e.g., stage 220 of FIG. 2, in some embodiments).

FIG. 4 shows a schematic diagram of a seeded ASE source 400 havingspectral filtering using a bulk diffraction grating spectral filter anda circulator. In some embodiments, the portion to the left of polarizer410 is identical to that of source 300 of FIG. 3, while in otherembodiments, an additional FBG 409 is added to the signal path to theleft of MUX 330, and Fiber Bragg grating FBG 340 between the left end ofthe Yb-doped fiber 350 and MUX 330 is omitted (e.g., the FBG 340 of FIG.3 is moved to the left-hand side of MUX 330 from the right-hand side).(In some embodiments, the portion to the left of polarizer 410 is usedfor block 210 of FIG. 2, and polarizer 410 and the portion to the rightof polarizer 410 is used for block 220 of FIG. 2.) In some embodiments,the output from non-polarization-maintaining isolator 360 is passedthrough a polarizer 410 and into polarization-maintaining circulator 420(which, in some embodiments, is a fairly lossy component). Some of thesignal then passes from circulator 420 to bulk grating 426 throughfiber-coupler/angle polished connector (FC/APC) 422 and achromat lens424 (which, in some embodiments, has a focal length of 35 mm). In someembodiments, bulk grating 426 is set at an angle to reflect 1080 nmlight back to circulator 420. The main output of circulator 420 thenpasses to PM amplifying fiber 450, which is pumped by laser diode 430(e.g., in some embodiments, a 180 mW, 975 nm laser diode) followed by afiber Bragg grating 432 to lock stability, and a fiber coupler 434, anda MUX 440 (which is used to combine the signal and pump light and/orminimize reflections). The linewidth-limited output of PM amplifyingfiber 450 is passed through PM isolator 460 and on to the next stage(e.g., block 230 of FIG. 2).

FIG. 5A shows a schematic diagram of a seeded ASE source 500 withdouble-pass amplifier using FBG filter with circulator. In someembodiments, the portion to the left of polarizer 510 is used in placeof source 300 of FIG. 3. In some embodiments, source 500 includes alaser diode pump 505 (e.g., in some embodiments, a 250 mW, 981.6 nmlaser diode) followed by a fiber Bragg grating 504 to lock stability,and a fiber coupler, and the pump output goes into single mode (SM)pump/signal multiplexer (MUX) 503. In some embodiments, MUX 503 outputspump light to ASE fiber 502. Fiber Bragg grating (FBG) 509 at the leftend of the ASE fiber source (Yb-doped fiber 502) is used to “seed” agiven wavelength (e.g., in some embodiments, it is highly reflective(HR) at 1080.7 nm with a linewidth of about 0.1 nm, thus providingenough feedback to fiber 502 to amplify at 1080.7 nm with a linewidth ofabout 0.1 nm, but this and the other parts are adjusted, if needed, toprevent lasing in fiber 502. In some embodiments, the output of fiber502 is passed through MUX 503 and through a non-polarization-maintainingisolator 506 and coupled to the next stage (e.g., to stage 220 of FIG.2, in some embodiments). Here, in some embodiments, the output fromnon-polarization-maintaining isolator 506 is passed through a polarizer510 and into polarization-maintaining circulator 520 (which, in someembodiments, is a fairly lossy component). Some of the signal thenpasses from circulator 520 to FBG filter 538 through MUX 535, whichmixes this signal with pump light from laser diode 530 (e.g., in someembodiments, a 250 mW, 981.6 nm laser diode) followed by a fiber Bragggrating 532 to lock stability, and a fiber coupler 534. Any lightexiting to the right of FBG 538 is dumped out angled cleave fiber end539. This combined pump and signal is amplified by Yb-doped PMamplifying fiber 536, is reflected by PM FBG 538 (which, in someembodiments, reflects 99% at 1080.7 nm with a 0.1 nm linewidth) thenpasses again through, and again is amplified by, Yb-doped PM amplifyingfiber 536. This reflected 1080 nm light goes back to circulator 520.

The main output of circulator 520 of FIG. 5A then passes to the nextstage (e.g., such as the following portions of FIG. 4: PM amplifyingfiber 450, which is pumped by laser diode 430 (e.g., in someembodiments, a 180 mW, 975 nm laser diode) followed by a fiber Bragggrating 432 to lock stability, and a fiber coupler 434, and a MUX 440(which is used to combine the signal and pump light and/or minimizereflections). The linewidth-limited output of PM amplifying fiber 450 ispassed through PM isolator 460 and on to the next stage (e.g., block 230of FIG. 2).

FIG. 5B shows a schematic diagram of an improved seeded ASE source 501with double-pass amplifier using FBG filter with circulator. In someembodiments, the portion to the left of polarizer 510 is used in placeof source 300 of FIG. 3. In some embodiments, source 501 includes alaser diode pump 505 (e.g., in some embodiments, a 250 mW, 981.6 nmlaser diode) followed by a fiber Bragg grating 504 to lock stability,and a fiber coupler, and the pump output goes into single mode (SM)pump/signal multiplexer (MUX) 503. In some embodiments, MUX 503 outputspump light to ASE fiber 502 (e.g., a type YB 164 fiber from INOcompany). Fiber Bragg grating (FBG) 509 at the left end of the ASE fibersource (Yb-doped fiber 502) is used to “seed” a given wavelength (e.g.,in some embodiments, it is highly reflective (HR) at 1080.7 nm with alinewidth of about 0.1 nm, thus providing enough feedback to fiber 502to amplify at 1080.7 nm with a linewidth of about 0.1 nm, but this andthe other parts are adjusted, if needed, to prevent lasing in fiber 502.Signal leaving the left end of FBG 509 exits an angled cleaves fiberend, to minimize reflections. In some embodiments, the fight-hand outputof fiber 502 is passed through MUX 503 and through anon-polarization-maintaining isolator 506 and coupled to the next stage(e.g., to stage 220 of FIG. 2, in some embodiments). Here, in someembodiments, the output from non-polarization-maintaining isolator 506is passed through a polarizer 510 and into circulator 520 (which, insome embodiments, is a fairly lossy component). Some of the signal thenpasses from circulator 520 to FBG filter 538 through MUX 534, whichmixes this signal with pump light from laser diode 530 (e.g., in someembodiments, a 250 mW, 981.6 nm laser diode) followed by a fiber Bragggrating 532 to lock stability, and a fiber coupler 534. Any lightexiting to the right of FBG 538 is dumped out angled cleave fiber end539. Combined pump and signal is amplified by Yb-doped PM amplifyingfiber 536 (e.g., a type YB 500 fiber from INO company), is reflected byPM FBG 538 (which, in some embodiments, reflects 99% at 1080.7 nm with a0.1 nm linewidth) then passes again through PM amplifying fiber 536 andagain is amplified by Yb-doped PM amplifying fiber 536. This reflected1080 nm light goes back to circulator 520.

The main output of circulator 520 of FIG. 5B then passes to the nextstage, starting with amplitude modulator 230 (which, in someembodiments, produces pulses about 100 ns long at a rate of 20,000pulses per second) and into circulator 540. Some of the signal thenpasses from circulator 540 to FBG filter 558 through MUX 554, whichmixes this signal with pump light from laser diode 550 (e.g., in someembodiments, a 250 mW, 975 nm laser diode) which is followed by a fiberBragg grating 552 to lock stability, and a fiber coupler 554. Any lightexiting to the right of FBG 558 is dumped out angled cleave fiber end559. Combined pump and signal is amplified by Yb-doped PM amplifyingfiber 556 (e.g., 8 meters of a type YB 500 fiber from INO (Sainte-FoyCANADA)—ino.zc.bmgmultimedia.com), is reflected by PM FBG 558 (which, insome embodiments, reflects 99% at 1080.7 nm with a 0.1 nm linewidth)then passes again through PM amplifying fiber 556 and again is amplifiedby Yb-doped PM amplifying fiber 556. This reflected 1080 nm light goesback to circulator 540, whose main output passes down and to the rightto amplifier 250. In some embodiments, amplifier 250 includes MUX 575,which mixes this signal with pump light from laser diode 570 (e.g., insome embodiments, a 500 mW, 975 nm laser diode) which is followed by afiber Bragg grating 572 to lock stability, and a fiber coupler 574. Thecombined pump and signal pass into Yb-doped PM amplifying fiber 556(e.g., 8 meters of a type YB 500 fiber from INO company) which amplifiesthe signal, which is then output to a next stage (e.g., a high power(e.g., 10 watts or more) large-mode-area gain fiber (e.g., as availablefrom, for example, Nufern (East Granby, Conn.)—see www.nufern.com)

Chirped Operation of a Single-Longitudinal-Mode Laser Diode

In cw operation, the linewidth of the single frequency laser diode is onthe order of a few MHz. The linewidth of the same laser diode in pulsedoperation is broadened significantly due to carrier inversion andthermal effects. In longer pulsed laser diodes (pulsewidth>5 nsec) thedominate cause of the broadening is thermal. The linewidth of the laserdiode will increase as the thermal load on the chip is increased untilthe mode—hop threshold has been surpassed.

As current is pulsed through the semiconductor material that defines thelaser cavity a significant amount of the electrical power is transformedinto heat. When the laser cavity is heated, the effective length of thecavity changes and as a result the discrete mode that the cavity cansupport changes. If the temperature and associated cavity length changesexcessively during the time the pulse is being generated the modesupported at the beginning of the pulse and the mode supported at theend of the pulse will change. The shift from one mode to another isreferred to as mode-hopping. Mode-hopping can have detrimentalconsequences in a laser or laser amplifier system but by carefullyselecting the maximum current, cw bias current and pulse durationmode-hopping can be eliminated.

Mode-hopping is caused by a significant change in the temperature of thelaser cavity while the pulse is being generated. In order to mitigateany mode hopping issues, we have taken steps to minimize the temperaturechange by minimizing the change in the drive current that act as theheat source during the pulse generation. The first step is to optimizethe cw bias current. The cw bias current should be set as high aspossible without exceeding the laser threshold current. The cw biascurrent heats the laser cavity during the time between pulses whichreduces the total change in the heat load the laser cavity willexperience when pulsed. The pulse duration is then set to the minimumpulse width required. By minimizing the time that the laser is driven atmaximum current the power and associated heat dumped into the lasercavity is also minimized. The third optimization point is the maximumdrive current. The maximum current is selected by adjusting the pulseddrive current level until mode-hopping is observed. The maximum currentis then reduced by some fraction (but still greater then the thresholdcurrent) to insure mode-hop free operation.

FIG. 12 illustrates an optical system 1200 for “pulse slicing”. In someembodiments, system 1200 is configured such that the output pulse fromthe single frequency laser diode 1210 is significantly longer than therequired output pulse. The long initial pulse 1212 is sent through a PMisolator 1216, coupler 1218 (e.g., a 99:1 ratio coupler), and through anoptical gate 1228 (e.g., in this case an acousto-optic modulator (AOM)),and a temporally selected slice or section of the original pulse istransmitted to create a shorter duration pulse of a specific timeportion and thus wavelength-chirp portion, since the original pulse ischirped) and into circulator 1220 (which, in some embodiments, is afairly lossy component). Some of the signal then passes from circulator1220 to FBG filter 538 through MUX 535, which mixes this signal withpump light from laser diode 1230 (e.g., in some embodiments, a 250 mW,981.6 μm laser diode) followed by a fiber Bragg grating 1232 to lockstability, and a fiber coupler 1234. This combined pump and signal isamplified by Yb-doped PM amplifying fiber 1236, is reflected by PM FBG1238 (which, in some embodiments, reflects 99% at 1080.7 nm with a 0.1nm linewidth, where any light exiting to the right of FBG 1238 is dumpedout angled cleave fiber end 1239) then passes again through, and againis amplified by, Yb-doped PM amplifying fiber 1236. This reflected 1080nm light goes back to circulator 1220.

The wavelength shift (chirp) of the laser diode is more rapid at thestart of the pulse and slows at later times, resulting in a nonlinearshift with time. Therefore “slices” of the original pulse have differentchirp amounts depending on the relative timing of the amplitudemodulator relative to the start of the original pulse. The wavelengthchirp gives an effective linewidth for the pulse that can be tuned byadjusting the relative timing of the sliced pulse relative to the startof the original pulse. For instance, if a 50-ns pulse duration isrequired, the wavelength change during the initial 50 ns of the original300 ns long pulse is greater than the last 50 ns. Therefore by adjustingthe AOM trigger relative to the laser diode pulse, the linewidth can betuned continuously. If a wider linewidth is desired, the optical gate(i.e. the AOM) is adjusted to open near the beginning of the diodepulse. Conversely if a shorter linewidth is preferred the AOM would beadjusted to open towards the end of the laser diode pulse. This allowsone to maintain the temporal pulsewidth but vary the linewidth of agiven laser diode source.

The time slicing technique can also be used to avoid mode hops that canoccur at a certain time in a pulse. For instance, if mode hops occurduring the rapid wavelength change at the beginning of the laser diodepulse, the optical gate can be adjusted to open after this time andinclude only the continuous wavelength change due to chirp without thediscontinuous mode hop.

The current invention includes two distinct applications. The first ofthese applications may involve the use of a band pass filter on a broadrange of frequencies to spectrally filter out a selected linewidth. Theother application is slightly more complicated and involves the use of alaser diode run at a current level that is “just above threshold.” Thisis the amount of current in a diode which causes the diode to changefrom absorbing light to amplifying light. At low currents, more light isabsorbed than emitted. For light emission to occur, there must be enoughspecies existing in the gain medium at an excited state. Once in thisstate, random spontaneous emission occurs. By adding current, the mediumremains at an excited state, resulting in the amplification of whateverfrequency was first emitted. As the laser diode turns on its centralfrequency will chirp, creating an initially broad linewidth whichnarrows over time. By selecting a small temporal portion of the chirp abroader or narrower linewidth will result.

Pulsed Characterization of the Laser Diode and Spectral Linewidth

One simple method to produce the appropriate linewidth is to directlymodulate the laser diode and utilize the chirped wavelength output asthe seed source for the amplifier system. We have demonstratedchirp-induced broadening of a DBR diode, with the goal of findingconditions that offer a broadened linewidth without mode hops. Wedemonstrated linewidths increased from less than 100 MHz (instrumentlimited) to ˜0.9 GHz in pulsed operation. In some embodiments, carefuladjustment of the peak current, pulse duration and operating temperaturewere necessary to avoid mode hops.

FIG. 6 shows the optical output of the diode. Several iterations of thepulsed current driver were tested to optimize performance andminiaturize the electronics circuit. FIG. 6 shows the most recentversion of the circuit which shows the rapid turn-on and turn-off times.The focus was on characterizing the diode laser performance justslightly above threshold to limit the chirp effects as much as possible.For the final system, the goal is to shape the current pulse tocounteract the effects of pulse steepening in the later amplifierstages.

Optical output pulse for one exemplary distributed Bragg-reflector (DBR)laser diode, available from the Sacher company (called the “Sacherdiode”), with a pulsed current source, as used in some embodiments. Inother embodiments, other suitable DBR laser diodes are used. For thisdata the peak drive current was set to approximately 75 mA and themeasured FWHM is 34 ns. The drive current and trigger pulsewidth can bevaried to change the pulse characteristics. The optical pulse shape canbe adjusted to compensate for pulse steepening in later amplifierstages.

In cw operation mode hops at specific current levels were seen. It wasdiscovered that in pulsed operation, the output showed multiple modes asseen in the OSA trace of FIG. 7. The Figure shows the optical outputwith 200 mA pulsed current. Attempts were made to reduce the peak drivecurrent to operate just barely above threshold to reduce the number ofmodes excited. FIG. 8 shows conditions where nearly single mode wasoperable, by limiting the current to 75 mA. This data shows excellentside mode suppression, although a more typical value was 10-12 dB.

FIG. 7 shows the output wavelength of the Sacher diode operating inpulsed mode with 200 mA peak drive current. Multiple longitudinal modesare present in the OSA data.

FIG. 8 shows the output wavelength of the Sacher diode operating inpulsed mode with the peak drive current set at 75 mA. Under certainconditions, the output could be limited to a single longitudinal modewith >20 dB side mode suppression.

The presence of additional modes was also visible in other data. FIG. 9shows the photodiode signal for the photodiode monitoring the totaloptical signal and the wavelength monitor photodiode measuring thetransmitted power through the etalon. The bandwidth of the etalon usedin the wavelength monitor in this example is 5.9 GHz. It appears thatthe laser diode operated in one mode for the first ˜5 ns of the pulse,then jumped to a second wavelength that was not transmitted by theetalon. After that jump the wavelength either jumped back to theoriginal nominal wavelength or to a third wavelength that happened to betransmitted by the etalon. This behavior was discovered to be associatedwith OSA spectra that showed multiple modes (similar to FIG. 7) withlimited side mode suppression.

FIG. 9 shows photodiode signals for the power monitor (Ch 3) and thewavelength monitor (Ch 2) after transmission through the second etalon.The etalon is tuned to the maximum transmission but the temporal signalindicates that the laser diode is operating at one wavelength for thefirst ˜5 ns, then jumps briefly to a second wavelength that is nottransmitted, and then to a third wavelength that is transmitted. Thisbehavior was correlated with OSA spectra that showed multiple modes inthe spectra with limited side mode suppression.

By adjusting the pulsed current to the gain section and the cw currentto the DBR section, the side mode suppression could be improved and thephotodiode signals shown in FIG. 10 could be obtained. Here the powerand wavelength monitor diodes show similar pulse shapes, and no sign ofmode-hopping. The difference in the DBR current between the two datasets was only 1.8 mA.

FIG. 10 shows photodiode signals for the power monitor (Ch 3) and thewavelength monitor (Ch 2) after transmission through the second etalon.The etalon is tuned to the maximum transmission for this data.

With the same conditions as those shown in FIG. 10, the linewidth wasmeasured to be 875 MHz. This is the average of the four peak widthsshown in FIG. 11.

FIG. 11 shows a scanning Fabry-Perot signal for the Sacher diodeoperating in pulsed mode with the peak drive current set at 75 mA and apulse duration of 35 ns. The average linewidth for the four peaks is 875MHz.

Tunable Linewidth Seed Source Setup

Since the linewidth of the seed source under simple pulsed operation wasstill too large for the specification, a slightly more complicated setupwas developed to generate pulsed output that provided variablelinewidth. The setup still used the chirp of the diode during a pulse,but used an acousto-optic modulator after the diode to select a timewindow. Since the diode chirps most rapidly at the beginning of thepulse, the diode could be pulsed for approximately 350-400 ns and theAOM could be timed to “slice” a 100 ns pulse at different delay times tocapture more or less of the diode chirp.

FIG. 12 is described above.

FIG. 13 shows diode drive current and relative timing of the AOM. Thediode current has a cw component to avoid mode hops, and a pulsedcomponent to provide a controlled amount of diode chirp. By timing theAOM pulse to slice a ˜100 ns section of the diode output, a selectionmay be made of either the rapid chirp at the beginning of the diodepulse to produce a large linewidth, or a relatively small chirp at theend of the diode pulse for narrower linewidth.

FIG. 14 shows scanning Fabry-Perot traces of the seed source linewidthfor various AOM delays.

FIG. 15 shows Seed source linewidth as a function of the AOM delay afterthe laser diode current pulse. By optimizing the cw and pulsed diodedrive currents, along with the relative AOM delay, the linewidth can beprogrammed from approximately 200 to 2000 MHz.

Apparatus and Method for Generating Temporally Shaped Laser-Seed-Signalsfor High-Powered Fiber-Laser Amplifier Systems

Some Limitations of Conventional Systems:

1. When a pulsed laser source is amplified the leading edge of the pulseis typically amplified to a greater extent than the trailing edge of thepulse. This is because the inversion, and therefore the gain, is higherbefore energy is extracted from the gain medium (when a gain medium ispumped to a high-level of inversion, the initial light is highlyamplified, however as the inversion energy is extracted and the laserintensity increases, there is a lower level of inversion and thisresults in less gain (later in time) to the laser output). This effecthas dramatic results when using rare-earth doped optical fibers toamplify a pulsed signal. (In a typical fiber amplifier the unextractedgain is very high, and the gain variation during the pulse can be verysubstantial.) A solution involves pre-distorting the pulse shape tocompensate for the gain variation during the pulse.

2. When a diode laser is turned on a very narrow gain-switched spike ispresent at the leading edge of the laser pulse. (Such gain-switchedspikes are well-known in a wide variety of laser systems. They happenwhen the gain is suddenly driven well above threshold, but there islittle light available to extract the gain. The light field can thenbuild up very rapidly, and produce a output spike before equilibriumbetween the pumping and extraction of the gain medium is established.)If amplified in a subsequent amplifier stage(s), the gain-switched spikecan cause optical damage in the laser system, and potentially cause poorperformance of the laser for its intended use. A solution involvestailoring the current drive to the diode laser to minimize the creationof the gain-switched spike.

3. For pulses of duration longer than several nanoseconds (say, longerthan about 5-10 ns), SBS (stimulated Brillouin scattering) limits theavailable peak power or energy available from pulsed fiber amplifiersystems with narrow-linewidth seed laser sources. A solution involvesmaking the optical frequency of the seed diode “chirp” (that is, slewrapidly) during the pulse.

4. In some applications, the shape, and not just the energy or durationof a laser pulse, has significant influence on the effects of the laserpulse. For example, in materials processing, it may be desirable togenerate a temporally flat-topped pulse rather than a sharply-peakedpulse, to avoid damage to a substrate from excessive peak power. Simplydriving the laser diode with a square or Gaussian-like pulse does notallow tailoring of the pulse shape.

In some embodiments of this invention, the drive current to thediode-laser seed source is controlled such that the optical pulse fromthe diode laser has an optimal shape for amplification in subsequentfiber amplifier stage(s). It is also possible to tailor the shape of theinput optical pulse to the fiber amplifier stages(s) to produce anoutput pulse whose shape is most desirable for subsequent uses, forexample, for materials processing. The invention includes the approachof pre-distorting the pulse to achieve the desired output pulse shape,tailoring the pulse shape for desired applications, techniques forsuppressing gain-switched spikes, and ramping the laser diode togenerate frequency chirp for SBS suppression.

In some embodiments, this is done by electronically controlling theshape of the drive current pulse applied to the seed laser diode in adiode laser—fiber amplifier system. In some embodiments, the laser diodedrive circuit sums several current sources that can be switched ontogether, or switched on sequentially with time delays between.Additionally, the current sources can produce a fixed current whenturned on, or in some cases a changing value. Because the gain of thefiber amplifier decreases as it is extracted during a pulse, it is oftendesirable for the seed source intensity to increase with time.Consequently, this design is capable of summing in one or moreincreasing ramp waveforms, whose slope and amplitude can be flexiblydefined.

FIG. 16A is a simplified schematic of a pulsed current source 1600 thatgenerates a very fast current pulse, where the current magnitude variesas a function of time, increasing the current later in the pulse tocompensate for the reduction in gain over time in a fiber gain medium.In some embodiments, current source 1600 includes a plurality of currentsources 1610, 1620, . . . 1630, each supplied from a different magnitudevoltage source V1, V2, . . . Vn, and each switched on at a successivelylonger delay, in order to increase the magnitude of the current pulseinto laser diode 1640 over time. Shorting switch 1650 ends the currentpulse into the laser diode 1640 after a suitable time delay t_(s), insome embodiments. Current source 1610 operates from voltage V1 1611,which flows through resistor 1613 once switch S1 1614 closes after adelay Δt₁, from start RAMP trigger 1605 (RAMP). Current source 1620operates from voltage V2 1621, which flows through resistor 1623 onceswitch S2 1624 closes after a delay Δt₂, from start RAMP trigger 1605(RAMP). Current source 1630 operates from voltage V1 1631, which flowsthrough resistor 1633 once switch Sn 1634 closes after a delay Δ_(n),from start RAMP trigger 1605 (RAMP).

FIG. 16B (which includes 16B1 and 16B2 and 16B3) includes a detailedschematic of one embodiment of pulsed current source 1600 that generatesa very fast current pulse, where the current magnitude varies as afunction of time, increasing the current later in the pulse tocompensate for the reduction in gain over time in a fiber gain medium.An external trigger source is applied simultaneously to multipleelements of the circuit. Each element (designated by the index n)includes a time delay Δt_(n), after which an electronic switch Sncloses. The switches Sn apply the various voltage sources V_(n) tocircuit elements L_(n) (inductors) or R_(n) (resistors). The currentspassing through elements L_(n) and R_(n) are combined and are applied tothe laser diode. Since it is desirable for many applications to achievea rapid fall-time at the end of the laser pulse, an additionalelectronic switch is provided to rapidly shunt the applied current toground, terminating the current flowing through the laser diode. FIG.16B3 is a simplified block diagram of one such component circuit element1601 that includes a circuit including series connected voltage sourceV1, switch S1, inductor L2 and a load.

The circuit provides great flexibility in the output waveform, since theapplied voltage, time delay, and series inductance or resistance foreach of several circuit elements can be varied independently.

In some embodiments, fiber amplifiers driven by a pulsed laser seedsource having a “square” pulse (a pulse that has substantially constantamplitude) will deplete their stored energy during the period of thepulse, thereby outputting an output pulse that has more power (i.e.,amplitude) at the beginning of the pulse (when the fiber amplifier hasthe most stored energy from the optical pump laser being converted tothe output wavelength) and less power towards the end of the pulse whenthe energy has been depleted by amplifying earlier portions of thepulse. This results in an output pulse that is “steeper” than the inputpulse. In some embodiments, in order to compensate for pulse steepeningeffects in the fiber amplifier, the temporal shape of the input pulse ismodified. In some embodiments, the input pulse has an amplitude that hasa rising slope, with the slope adjusted to the energy-depletion curve ofthe fiber amplifier, in order to obtain an output pulse having a desiredshape (e.g., in some embodiments, an output pulse having a more-constantamplitude). Typically a rising-slope waveform is used to compensate fordecreasing fiber amplifier gain over the duration of a pulse.

In some embodiments, to minimize the gain-switched spike, prior toproducing the desired pulse output, the diode laser drive current isturned on to a level slightly below the lasing threshold, or else rampedup slowly. By using either the sub-threshold excitation or the slowramp, the radiation field in the diode laser is established, minimizingor eliminating the gain-switched spike when the main current pulse isapplied to produce an optical pulse.

In cases where it is necessary to suppress SBS (for example, for pulselengths longer than about 5-10 ns) ramping the drive current causes thediode to frequency chirp. This increases the effective spectrallinewidth of the source, or, equivalently, decreases the dwell time atany specific wavelength, thwarting the build-up of SBS. This effect isparticularly useful with DFB (distributed feedback) or DBR (distributedBragg reflector) diodes, which are inherently narrowband devices. TheDFB and DBR devices exhibit significant frequency chirp as their drivecurrent is ramped during a pulse.

A further benefit of pre-shaping the seed diode pulse is to allowgeneration of pulse shapes optimized for a particular process, forexample, materials processing. By using a diode laser-fiber amplifiersystem with a flexible pulse-shaping apparatus, the performance for aparticular application can be optimized by adjusting the pulse shape.For example, Smart describes the usefulness of pulse shaping in U.S.Pat. No. 6,281,471 (which is incorporated herein by reference. However,this disclosure specifically describes generating the ultimately desiredpulse shape by modulating the seed laser, then amplifying the desiredpulse shape in a nondistorting amplifier. Our invention recognizes thatdistortion in fiber amplifiers is very common, and it is much morepractical to pre-compensate for these distortions with the versatilecircuit described here.

FIG. 16C is a graph of the shape of a shaped laser diode current pulsefor a seed source as a function of time, for five different pulse typesand current magnitudes. FIG. 17 is a graph of the spectral trace of aseed-source laser driven by each of the five pulses of FIG. 16. FIG. 18is a graph of the laser diode optical output as a function of time whendriven by a current pulse which increases with time, and the opticaloutput after passing through a fiber amplifier.

FIG. 19 is a graph of the shape of a laser pulse generated with a drivertypical of the prior art. It shows a prominent gain-switched spike atits start.

Some embodiments of the present invention include a filtered ASE sourcecoupled to a fiber laser amplifier for applications where the linewidthmust be greater than about 0.01 nm and controlled to be a fixed valuethat does not vary with laser properties, and having a linewidth lessthan about 10 nm.

Some embodiments of the present invention include a filtered ASE sourcefor a polarization maintaining (PM) fiber laser amplifier.

Some embodiments of the present invention include a filtered ASE sourcefor an LMA fiber laser amplifier.

Some embodiments of the present invention include a filtered ASE sourcefor a PM LMA fiber laser amplifier.

Some embodiments of the present invention include a filtered ASE sourcefollowed by modulator as a seed source for a fiber amplifier with pulsedoutput.

Some embodiments of the present invention include a filtered ASE sourcefollowed by modulator as a seed source for a PM fiber amplifier withpulsed output.

Some embodiments of the present invention include a filtered ASE sourcefollowed by modulator as a seed source for an LMA fiber amplifier withpulsed output.

Some embodiments of the present invention include a filtered ASE sourcefollowed by modulator as a seed source for a PM LMA fiber amplifier withpulsed output.

Some embodiments of the present invention include a chirped laser diodeas seed source for fiber laser amplifier.

Some embodiments of the present invention include a chirped singlelongitudinal mode laser diode as a seed source for a fiber laseramplifier.

Some embodiments of the present invention include a chirped singlelongitudinal mode laser diode as a seed source for a PM fiber laseramplifier.

Some embodiments of the present invention include a chirped singlelongitudinal mode laser diode as a seed source for a LMA fiber laseramplifier.

Some embodiments of the present invention include a chirped singlelongitudinal mode laser diode as a seed source for a PM LMA fiber laseramplifier.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering andless than about 1 nm to be within the spectral acceptance of a nonlinearoptical crystal.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering and less than about 1 nm to bewithin the spectral acceptance of a nonlinear optical crystal.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering andthe amplifier is substantially polarization-maintaining.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering andless than about 1 nm to be within the spectral acceptance of a nonlinearoptical crystal and the amplifier is substantiallypolarization-maintaining.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering and the amplifier issubstantially polarization-maintaining.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering and less than about 1 nm to bewithin the spectral acceptance of a nonlinear optical crystal and theamplifier is substantially polarization-maintaining.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering,followed by a modulator, as seed source for fiber amplifier with pulsedoutput.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering andless than about 1 nm to be within the spectral acceptance of a nonlinearoptical crystal, followed by a modulator, as seed source for fiberamplifier with pulsed output.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering, followed by a modulator, asseed source for fiber amplifier with pulsed output.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering and less than about 1 nm to bewithin the spectral acceptance of a nonlinear optical crystal, followedby a modulator, as seed source for fiber amplifier with pulsed output.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering,followed by a modulator, as seed source for fiber amplifier with pulsedoutput where the amplifier is substantially polarization-maintaining.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by awavelength dependent filter for applications where the linewidth must begreater than about 0.01 nm to avoid Stimulated Brillouin Scattering andless than about 1 nm to be within the spectral acceptance of a nonlinearoptical crystal, followed by a modulator, as seed source for fiberamplifier with pulsed output where the amplifier is substantiallypolarization-maintaining.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering, followed by a modulator, asseed source for fiber amplifier with pulsed output where the amplifieris substantially polarization-maintaining.

Some embodiments of the present invention include an optical seed sourcefor fiber laser amplifiers where the linewidth is determined by an FBGfor applications where the linewidth must be greater than about 0.01 nmto avoid Stimulated Brillouin Scattering and less than about 1 nm to bewithin the spectral acceptance of a nonlinear optical crystal, followedby a modulator, as seed source for fiber amplifier with pulsed outputwhere the amplifier is substantially polarization-maintaining.

Some embodiments of the present invention include a filtered ASE sourcefollowed by modulator as seed source for LMA fiber amplifier with pulsedoutput.

Some embodiments of the present invention include a filtered ASE sourcefollowed by modulator as seed source for PM LMA fiber amplifier withpulsed output.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Although numerous characteristics andadvantages of various embodiments as described herein have been setforth in the foregoing description, together with details of thestructure and function of various embodiments, many other embodimentsand changes to details will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention shouldbe, therefore, determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein,” respectively. Moreover, the terms “first,” “second,” and“third,” etc., are used merely as labels, and are not intended to imposenumerical requirements on their objects.

1. An apparatus comprising: a source of ASE seed light, wherein theseed-light source includes: a first source of pump light; a firstoptically pumped fiber amplifier operatively coupled to receive the pumplight and configured to generate and outputs onlyamplified-spontaneous-emission (ASE) light; a first filter operativelycoupled to filter ASE light of the first fiber amplifier such that theseed-light source generates ASE seed light having a controlledsubstantially fixed linewidth of between about 0.01 nm and about 10 nm;and a second optically pumped fiber amplifier operatively coupled toreceive, amplify, and filter the filtered ASE light from the seed-lightsource and to output amplified filtered ASE light and not configured tolase.
 2. An apparatus comprising: a source of ASE seed light, whereinthe seed-light source includes: a first source of pump light; a firstoptically pumped fiber amplifier operatively coupled to receive the pumplight and configured to generate amplified-spontaneous-emission (ASE)light; a first filter operatively coupled to filter ASE light of thefirst fiber amplifier such that the seed-light source generates ASE seedlight having a controlled substantially fixed linewidth of between about0.01 nm and about 10 nm; and a second optically pumped fiber amplifieroperatively coupled to receive, amplify, and filter the filtered ASElight from the seed-light source and to output amplified filtered ASElight and not configured to lase, wherein the second fiber amplifierincludes a polarization-maintaining (PM) fiber amplifier operativelycoupled to receive and amplify light generated by the seed source.
 3. Anapparatus comprising: a source of ASE seed light, wherein the seed-lightsource includes: a first source of pump light; a first optically pumpedfiber amplifier operatively coupled to receive the pump light andconfigured to generate amplified-spontaneous-emission (ASE) light; afirst filter operatively coupled to filter ASE light of the first fiberamplifier such that the seed-light source generates ASE seed lighthaving a controlled substantially fixed linewidth of between about 0.01nm and about 10 nm; and a second optically pumped fiber amplifieroperatively coupled to receive, amplify, and filter the filtered ASElight from the seed-light source and to output amplified filtered ASElight and not configured to lase, wherein the first filter includes afiber-Bragg-grating (FBG), wherein a first end of the FBG is opticallycoupled to a first end of the first fiber amplifier, wherein the FBG ishighly reflective for light within the linewidth of the seed light suchthat filtered seed light is reflected toward the first fiber amplifier,and wherein a second end of the FBG is optically coupled to a light dumpthat substantially prevents reflections of light back towards the FBG.4. The apparatus of claim 3, wherein the seed-light source furtherincludes a first optical isolator optically coupled to a second end ofthe first fiber amplifier such that the seed light is output through thefirst optical isolator.
 5. The apparatus of claim 4, wherein theseed-light source further includes a first optical multiplexer (mux)optically coupled between the first filter and the light dump and thefirst pump-light source such that at least part of the light that ispassed from the first end of the first fiber amplifier through the firstfilter passes through the first mux and to the light dump, while pumplight is passed from the first pump-light source through the first muxand the first filter into the first end of the first fiber amplifier. 6.The apparatus of claim 5, further including a first pump-light filteroperatively coupled between the first optical isolator and the firstpump-light source, wherein the pump-light filter passes light having awavelength of the pump light and blocks light having a wavelength of theseed light.
 7. The apparatus of claim 3, wherein the seed-light sourcefurther includes a first optical multiplexer (mux) optically coupledbetween the first end of the first fiber amplifier and the first filterand the first pump-light source such that the seed light is passed fromthe first end of the first fiber amplifier through the first mux and tothe first filter, while pump light is passed from the first pump-lightsource through the first mux and into the first end of the first fiberamplifier.
 8. The apparatus of claim 7, further including a firstpump-light filter operatively coupled between the first optical isolatorand the first pump-light source, wherein the pump-light filter passeslight having a wavelength of the pump light and blocks light having awavelength of the seed light.
 9. The apparatus of claim 3, wherein theseed-light source further includes a first optical multiplexer (mux)optically coupled between the second end of the first fiber amplifierand the first optical isolator and the first pump-light source such thatthe seed light is passed from the second end of the first fiberamplifier through the first mux and into the first optical isolator,while pump light is passed from the first pump-light source through thefirst mux and into the second end of the first fiber amplifier.
 10. Theapparatus of claim 9, further including a first pump-light filteroperatively coupled between the first optical isolator and the firstpump-light source, wherein the pump-light filter passes light having awavelength of the pump light and blocks light having a wavelength of theseed light.
 11. The apparatus of claim 4, wherein the seed-light sourcefurther includes a polarizer optically coupled to receive seed lightfrom the first isolator and to output polarized seed light.
 12. Anapparatus comprising: a source of ASE seed light, wherein the seed-lightsource includes: a first source of pump light; a first optically pumpedfiber amplifier operatively coupled to receive the pump light andconfigured to generate amplified-spontaneous-emission (ASE) light; and afirst filter operatively coupled to filter ASE light of the first fiberamplifier such that the seed-light source generates ASE seed lighthaving a controlled substantially fixed linewidth of between about 0.01nm and about 10 nm; wherein the first fiber amplifier includes Ybdoping, wherein the first filter includes a fiber-Bragg-grating (FBG),wherein a first end of the FBG is optically coupled to a first end ofthe first fiber amplifier, wherein the FBG is highly reflective forlight having a wavelength of about 1080 nm and a linewidth of about 0.1nm such that filtered seed light is reflected toward the first fiberamplifier, and wherein a second end of the FBG is optically coupled toan angle-cleaved light dump that prevents substantially all reflectionsof light back towards the FBG; and wherein the seed-light source furtherincludes: a first optical isolator optically coupled to a second end ofthe first fiber amplifier such that the seed light is output through thefirst optical isolator; a first optical multiplexer (mux) opticallycoupled between the second end of the first fiber amplifier and thefirst optical isolator and the first pump-light source such that theseed light is passed from the second end of the first fiber amplifierthrough the first mux and into the first optical isolator, while pumplight having a wavelength of about 980 nm is passed from the firstpump-light source through the first mux and into the second end of thefirst fiber amplifier; and a first pump-light filter operatively coupledbetween the first optical isolator and the first pump-light source,wherein the pump-light filter passes light having a wavelength of about980 nm and blocks light having a wavelength of about 1080 nm.
 13. Theapparatus of claim 4, further comprising: a polarizer optically coupledto receive seed light from the first isolator and to output polarizedseed light; and a polarization-maintaining (PM) fiber amplifieroperatively coupled to receive and configured to amplify light generatedby the seed-light source.
 14. An apparatus comprising: a source of ASEseed light, wherein the seed-light source includes: a first source ofpump light; a first optically pumped fiber amplifier operatively coupledto receive the pump light and configured to generateamplified-spontaneous-emission (ASE) light; and a first filteroperatively coupled to filter ASE light of the first fiber amplifiersuch that the seed-light source generates ASE seed light having acontrolled substantially fixed linewidth of between about 0.01 nm andabout 10 nm; wherein the first filter includes a fiber-Bragg-grating(FBG), wherein a first end of the FBG is optically coupled to a firstend of the first fiber amplifier, wherein the FBG is highly reflectivefor light within the linewidth of the seed light such that filtered seedlight is reflected toward the first fiber amplifier, and wherein asecond end of the FBG is optically coupled to a light dump thatsubstantially prevents reflections of light back towards the FBG;wherein the seed-light source further includes a first optical isolatoroptically coupled to a second end of the first fiber amplifier such thatthe seed light is output through the first optical isolator; a polarizeroptically coupled to receive seed light from the first isolator and tooutput polarized seed light; and a polarization-maintaining (PM) fiberamplifier operatively coupled to receive and configured to amplify lightgenerated by the seed-light source; wherein the PM fiber amplifiercomprises: a second source of pump light; a second optically pumpedfiber amplifier operatively coupled to receive the pump light from thesecond pump-light source and configured to amplify ASE light signals; asecond FBG operatively coupled to a first end of the second fiberamplifier, the second FBG configured to be highly reflective of light ofa wavelength of the seed light and to pass other wavelengths; and afirst circulator having a first port coupled to receive the polarizedseed light from the polarizer, a second port optically coupled to asecond end of the second fiber amplifier, and a third port configured tooutput amplified polarized seed light.
 15. An apparatus comprising: asource of ASE seed light, wherein the seed-light source includes: afirst source of pump light; a first optically pumped fiber amplifieroperatively coupled to receive the pump light and configured to generateamplified-spontaneous-emission (ASE) light; and a first filteroperatively coupled to filter ASE light of the first fiber amplifiersuch that the seed-light source generates ASE seed light having acontrolled substantially fixed linewidth of between about 0.01 nm andabout 10 nm; wherein the first filter includes a fiber-Bragg-grating(FBG), wherein a first end of the FBG is optically coupled to a firstend of the first fiber amplifier, wherein the FBG is highly reflectivefor light within the linewidth of the seed light such that filtered seedlight is reflected toward the first fiber amplifier, and wherein asecond end of the FBG is optically coupled to a light dump thatsubstantially prevents reflections of light back towards the FBG;wherein the seed-light source further includes a first optical isolatoroptically coupled to a second end of the first fiber amplifier such thatthe seed light is output through the first optical isolator; theapparatus further comprising: a polarizer optically coupled to receiveseed light from the first isolator and to output polarized seed light;and a polarization-maintaining (PM) fiber amplifier operatively coupledto receive and configured to amplify light generated by the seed-lightsource; wherein the PM fiber amplifier comprises: a second source ofpump light; a second optically pumped fiber amplifier operativelycoupled to receive the pump light from the second pump-light source andconfigured to amplify ASE light signals; a second FBG operativelycoupled to a first end of the second fiber amplifier, the second FBGconfigured to be highly reflective of light of a wavelength of the seedlight and to pass other wavelengths; a first circulator having a firstport coupled to receive the polarized seed light from the polarizer, asecond port optically coupled to a second end of the second fiberamplifier, and a third port configured to output amplified polarizedseed light; wherein the PM fiber amplifier further comprises: anamplitude modulator optically coupled to receive amplified seed lightfrom the first circulator and to output amplitude-modulated seed light;a third source of pump light; a third optically pumped fiber amplifieroperatively coupled to receive the pump light from the third pump-lightsource and configured to amplify ASE light signals; a third FBGoperatively coupled to a first end of the third fiber amplifier, thethird FBG configured to be highly reflective of light of a wavelength ofthe seed light and to pass other wavelengths; a second circulator havinga first port coupled to receive the amplitude-modulated polarized seedlight from the amplitude modulator, a second port optically coupled to asecond end of the third fiber amplifier, and a third port configured tooutput amplified amplitude-modulated polarized seed light; a fourthsource of pump light; a fourth optically pumped fiber amplifieroperatively coupled to receive the pump light from the fourth pump-lightsource and configured to receive amplified amplitude-modulated polarizedseed light from the second circulator and to output amplifiedamplitude-modulated polarized seed light, wherein the second, third, andfourth fiber amplifiers include polarization-maintaining fibers, and thesecond, third, and fourth FBGs include polarization-maintaining FBGs.16. An apparatus comprising: a source of ASE seed light, wherein theseed-light source includes: a first source of pump light; a firstoptically pumped fiber amplifier operatively coupled to receive the pumplight and configured to generate amplified-spontaneous-emission (ASE)light; and a first filter operatively coupled to filter ASE light of thefirst fiber amplifier such that the seed-light source generates ASE seedlight having a controlled substantially fixed linewidth of between about0.01 nm and about 10 nm; wherein the first filter includes afiber-Bragg-grating (FBG), wherein a first end of the FBG is opticallycoupled to a first end of the first fiber amplifier, wherein the FBG ishighly reflective for light within the linewidth of the seed light suchthat filtered seed light is reflected toward the first fiber amplifier,and wherein a second end of the FBG is optically coupled to a light dumpthat substantially prevents reflections of light back towards the FBG;wherein the seed-light source further includes a first optical isolatoroptically coupled to a second end of the first fiber amplifier such thatthe seed light is output through the first optical isolator; a polarizeroptically coupled to receive seed light from the first isolator and tooutput polarized seed light; and a polarization-maintaining (PM) fiberamplifier operatively coupled to receive and configured to amplify lightgenerated by the seed-light source; wherein the PM fiber amplifiercomprises: a fiber section and a lens operatively coupled to generate ina first direction a collimated light beam from light emitted from thefiber and to receive in an opposite second direction a filteredcollimated light and inject the filtered light back into the fiber; adiffraction grating configured to be highly retro-reflective of light inthe collimated beam of a wavelength of the seed light and to reflectlight of other wavelengths in other directions; a third circulatorhaving a first port coupled to receive the polarized seed light from thepolarizer, a second port optically coupled to the fiber section, and athird port configured to output further-filtered polarized seed light; afifth source of pump light; and a fifth optically pumped fiber amplifieroperatively coupled to receive the pump light from the fifth pump-lightsource and configured to receive further-filtered polarized seed lightfrom the third circulator and to output amplified further-filteredpolarized seed light, wherein the fifth fiber amplifier includes apolarization-maintaining fiber.
 17. An apparatus comprising: a fiberamplifier, wherein the fiber amplifier is not configured to lase; andfiltered-amplified-spontaneous-emission (ASE)-seed means for preventingstimulated Brillouin scattering (SBS) buildup in the fiber amplifier,wherein the ASE-seed means outputs filtered ASE light having asubstantially fixed linewidth of between about 0.01 nm and about 10 nm.18. An apparatus comprising: a fiber amplifier, wherein the fiberamplifier is not configured to lase; andfiltered-amplified-spontaneous-emission (ASE)-seed means for preventingstimulated Brillouin scattering (SBS) buildup in the fiber amplifier,wherein the filtered-ASE-seed means comprises: an optical isolator; afirst fiber Bragg grating (FBG) that is highly reflective within anarrow linewidth and configured to dump light outside the narrowlinewidth; and a first amplifier fiber operatively coupled between theoptical isolator and the first FBG and configured to amplify filteredASE light reflected from the first FBG.
 19. An apparatus comprising: afiber amplifier, wherein the fiber amplifier is not configured to lase;and filtered-amplified-spontaneous-emission (ASE)-seed means forpreventing stimulated Brillouin scattering (SBS) buildup in the fiberamplifier, wherein the fiber amplifier is polarization maintaining andthe filtered-ASE-seed means comprises: an optical isolator; a firstfiber Bragg grating (FBG) that is highly reflective within a narrowlinewidth and configured to dump light outside the narrow linewidth; afirst amplifier fiber operatively coupled between the optical isolatorand the first FBG and configured to amplify filtered ASE light reflectedfrom the first FBG; and a polarizer optically coupled to the opticalisolator to provide a filtered polarized ASE seed signal to the fiberamplifier.
 20. A method comprising: providing a fiber amplifier, whereinthe fiber amplifier is not configured to lase; and preventing stimulatedBrillouin scattering (SBS) buildup in the fiber amplifier by using aseed-light source of amplified-spontaneous-emission (ASE) light andfiltering the ASE light to have a substantially fixed linewidth ofbetween about 0.01 nm and about 10 nm.
 21. The apparatus of claim 1,wherein the seed light is controlled to have a linewidth of about 0.1nm.
 22. The apparatus of claim 1, wherein the first fiber amplifierincludes Yb doping.
 23. The apparatus of claim 17, wherein thefiltered-amplified-spontaneous-emission (ASE)-seed means outputs lighthaving a linewidth of about 0.1 nm.
 24. The apparatus of claim 17,wherein the first fiber amplifier includes Yb doping.
 25. The method ofclaim 20, wherein the filtering includes filtering the ASE light to havea linewidth of about 0.1 nm.
 26. The method of claim 20, whereinproviding of the first fiber amplifier includes providing the firstfiber amplifier with Yb doping.